Such apparatuses are well-known in the art, examples of which are described below.
U.S. Pat. No. 4,536,757, owned by Philips Electronics and herein incorporated by reference, discloses a device for signaling a specific charge condition of an accumulator battery incorporated in a battery-powered apparatus, such as a shaver equipped with a rechargeable battery. The device comprises a comparator circuit for comparing the battery voltage with a reference voltage, a switching circuit controlled by the comparator circuit, and an indicator controlled by the switching circuit. The signaling point, i.e. the charging level of the battery at which the switching circuit responds, may be selected so that, at the instant of signaling, the battery charge is still sufficient for a complete shave.
U.S. Pat. No. 5,973,497, incorporated herein by reference, discloses a method of determining and displaying the charge status of a storage battery of a battery-operated appliance, such as a razor. A time-based charge status is calculated, based at least in part on the cumulative operating time of the appliance since recharging. The battery terminal voltage is also monitored. The time-based charge status is displayed until the battery voltage indicates a low voltage condition, at which point a low-charge alert indicator is displayed only if the time-based charge status is below a predetermined level (i.e. if the cumulative operating time is above a predetermined level). Methods of detecting battery ageing and accounting for periods of excessive current flow are included. Some embodiments include the steps of waiting for a preset period of time, once the low-charge alert indicator is displayed, and then displaying a “zero”-charge status and disconnecting the power-consuming device from the storage battery unit by means of a controllable switching device. By automatically disconnecting the power-consuming device from the storage battery, the function of the appliance is suspended demonstrably to the user by draining the storage battery at a defined moment, while the storage battery is subjected to less strain when it is not deep-discharged to complete exhaustion. This residual charge of the storage battery enables components such as microprocessors or ASICs, which require a minimum voltage to perform certain basic functions, to run for some extra time, which may amount to several weeks. The power-consuming device is preferably switched off by decreasing the power consumption of the power-consuming device to zero over an extended period of time. This is advantageous, for example, with an electric shaver or hair cutter because the risk of hairs being caught by suddenly switching off the appliance can be eliminated. The reduction of speed may extend over several seconds.
U.S. Pat. No. 5,623,193, incorporated herein by reference, discloses an electric device in which an electric load is powered by a battery via an electronic switching device. A display means serves the function of indicating the charge status of the battery, and a time measurement means determines the connection time of the load to the battery. Following expiration of a predetermined connection time of the load to the battery, when the display means indicates the “zero”-charge status, the time measurement means will disconnect the electric load from the battery by means of the electronic switching device.